Field of the Invention
The present invention relates to a technique of generating an image signal for image display by a liquid crystal display element.
Description of the Related Art
Liquid crystal display elements are used for many display apparatuses, such as direct-view monitors and liquid crystal projectors, each displaying images. FIG. 12 illustrates a schematic configuration of a liquid crystal display element. Between a common electrode 1201 and each of multiple pixel electrodes 1202, a liquid crystal containing a liquid crystal molecule 1203 is disposed. A voltage to be applied to each pixel electrode 1202 (that is, a potential difference between the common electrode 1201 and each pixel electrodes 1202) is changed depending on a tone of the image signal. Changing the applied voltage enables controlling a direction of the liquid crystal molecule 1203, which enables controlling a light amount (i.e., a display tone) of light exiting from the pixel containing the liquid crystal molecule 1203. Controlling the direction of the liquid crystal molecule 1203 of each of multiple pixels of the liquid crystal display element enables displaying an image.
The direction of the liquid crystal molecule 1203 is defined by a polar angle θ and an azimuth angle Φ in a spherical coordinate system illustrated in FIG. 12. The polar angle θ is changeable depending on the potential difference (absolute value) between the common electrode 1201 and the pixel electrode 1202. In the liquid crystal display element being in a so-called normally black mode, an increase in the potential difference increases the polar angle θ and heightens the display tone.
On the other hand, in the normally black mode, the azimuth angle Φ becomes a specific angle (pre-tilt azimuth) due to a weak alignment-regulating force caused by an alignment film formed on surfaces of the common electrode 1201 and the pixel electrode 1202.
However, the liquid crystal display element has commonly known problems, namely, unevenness in alignment of the liquid crystal molecules, which is so-called disclination, and a decrease in image quality due to the disclination. FIG. 9 illustrates an example of generation of the disclination. When an image containing a white background and a black line 201 extending vertically is displayed as illustrated in FIG. 9, a dark line (disclination line) 202 due to the disclination, which is caused by a difference in a potential (driving voltage) between mutually adjacent pixels, is generated in pixels adjacent to the right of the pixels of the black line 201. With reference to FIG. 10A, description will be made of the directions of liquid crystal molecules of a pixel 200 in which the disclination is generated.
In FIG. 10A, multiple liquid crystal molecules 204 contained in the pixel 200 in which the disclination is generated have a pre-tilt azimuth set by the alignment film formed on the surface of the electrode such that the molecules 204 are oriented in a pre-tilt direction 206 expressed by a line connecting an upper left part and a lower right part of the drawing. In addition, the polar angle of each liquid crystal molecule 204 that is an angle formed with respect to a normal of a plane of the drawing changes depending on the driving voltage, which provides tones from black to white. The drawing illustrates an example of a negative liquid crystal whose liquid crystal molecules 204 are oriented in a direction vertical to the plane of the drawing in a state in which the driving voltage is not applied and are oriented in a direction parallel to the plane of the drawing (and in the pre-tilt direction 206) in a state in which the driving voltage is applied.
Multiple liquid crystal molecules 205 located in an area in the pixel 200 adjacent to the pixel (black voltage applied pixel) displaying the black line 201 in FIG. 9 are affected by the potential difference from the black voltage applied pixel and thus are oriented in an direction 207 different from the pre-tilt direction 206 (that is, a direction parallel to a vertical side of the pixel 200). Consequently, as illustrated in FIG. 9, the disclination line 202 is generated in the pixel 200.
Japanese Patent Laid-Open No. 2012-203052 discloses an image processing method of decreasing a difference in a tone level of a target pixel from that of an adjacent pixel in order to suppress the generation of the disclination in the target pixel.
In addition, a condition of the generation of the disclination depends not only on a magnitude of the potential difference of the target pixel from the adjacent pixel, but also on a relation between a direction of a gradient of the potential difference and the pre-tilt azimuth. A pixel 203 illustrated in FIG. 9 is a pixel whose sign of the gradient of the potential difference with respect to the pre-tilt direction 206 is inverse to that of the pixel 200. In the above-described pixel 203, as illustrated in FIG. 10B, though a direction 209 of liquid crystal molecules 208 slightly changes with respect to the pre-tilt direction 206 illustrated in FIG. 10A due to an influence of the potential difference of the pixel 203 from the adjacent pixel, the direction 209 does not become parallel to the vertical side of the pixel 203. For this reason, the disclination is not generated in the pixel 203.
Furthermore, displaying on the liquid crystal display element a moving image whose sequential frame images are images in which the disclination is generated results in unevenness in image quality that is so-called a tailing. FIG. 11 illustrates a state in which a tailing 213 is generated in a moving image (between multiple frame images) containing a white background and a black rectangle. At a right side and a lower side of the black rectangle, disclination lines 211 and 212 are generated. When the black rectangle moves in a direction in which the disclination remain, a temporal residue of the disclination in its reducing process seems like a tail. In particular, when, as indicated by a white-filled arrow in the drawing, a movement direction of the black rectangle is an oblique direction opposite to a convex direction of a white-background side corner portion of the black rectangle (that is, an upper left oblique direction in the drawing), the tailing 213 appears.
The method disclosed in Japanese Patent Laid-Open No. 2012-203052 enables suppressing the generation of the disclination, thereby suppressing the generation of the tailing due to the disclination. However, use of the method disclosed in Japanese Patent Laid-Open No. 2012-203052 is likely to decrease a brightness and a contrast of a displayed image.